1. Field of the Invention
The general field of this invention is tomography, that field that relates to obtaining an image of internal body parts in a plane through the body. Specifically, the field of this invention, called transverse axial tomography, relates to a method and apparatus for calculating the value of physical characteristic at a point within a body from the values of a plurality of line integrals derived from an incoherent propagation process through the body. The invention is particularly suited for applying a plurality of X-ray or gamma ray beams through a plane of a body, measuring the attenuation of each beam as it passes through the body, and using the measurement information obtained to construct individual attenuation coefficients for each element of a defined element matrix in the body plane.
2. Description of the Prior Art
A prior art method and apparatus for transverse axial tomography is described in U.S. Pat. No. 3,778,614 issued Dec. 1, 1973. That patent describes a technique to reconstruct a cross-sectional view of a body from a series of transmission measurements obtained by translating a radiation source and detector across the body section and repeating this translation motion at a number of angular orientations in the plane of the section. U.S. Pat. No. 3,778,614 is incorporated herein, by reference, as background material.
The objective of these measurements is to obtain, after computer analysis of thousands of pieces of raw information about beam attenuation through the body plane, the attenuation coefficient associated with each element of a matrix defined in the body plane. The method is useful for internal description of any body, but is primarily useful for identification of internal human body abnormalities. The attenuation coefficients are different for normal body tissue, tumors, fat, etc. and consequently provide identifying information about soft tissues in a human body. Especially useful for identification of brain disease and abnormalities, tomography by computer reconstruction eliminates obvious disadvantages of patient discomfort and morbidity normally associated with brain investigations using pneumography, angiography and radioactive isotope scanning.
In one prior art method, the scan signals are processed to yield visual information and local values of the beam attenuation coefficients over the body section. Detector scan signals are applied to an analog/digital converter to convert the analog scan signals which are proportional to each beam attenuation to digital form and subsequently are recorded in a storage unit. Computer analysis of the entire matrix of scan signals, typically about 28,000 points, yields attenuation coefficients associated with a element matrix defined for the body. These attenuation coefficients are related to the local physical properties in the body plane. After they are computed, the attenuation coefficients are recorded in a storage unit, and subsequently converted to analog signals by means of a digital/analog converter. These signals drive a viewing unit, typically a CRT, with the information content to pictorially display the attenuation coefficient for each matrix element. A permanent record of the display is achieved by means of a camera.
Another prior art method for tomographic image reconstruction makes use of a convolution of backprojection algorithm; as described, for example, in U.S. Pat. No. 3,924,129 which is incorporated herein, by reference, as background material.
High spatial frequency components associated with X-ray scanning measurements can contribute to the production of artifacts in reconstructed images. U.S. Pat. No. 4,002,911 describes a tomographic scanner wherein the intensity profile of an X-ray beam is weighted to limit high frequency components. Weighting of the X-ray beam intensity profile is mathematically equivalent to the inclusion of a weighting function in the convolution integral but is necessarily limited by physical constraints of the X-ray system (i.e. weighting of an X-ray beam is limited to positive weighting functions).
At the present time substantial interest exists in the use of the methods and apparatus of computed tomography for the reconstruction of X-ray images of the human body. The computational methods utilized are, however, equally applicable to other fields wherein the local characteristic values must be calculated from line integrals derived from an incoherent propagation process. As used herein, the term "incoherent propagation" means that the value of the characteristic at each point affects the value of the integral in a manner which is uncorrelated to the value of the characteristic at other points. Although the method and apparatus for the invention are described herein with respect to X-ray scanning apparatus, the use of the invention is by no means limited to that field.
A region of interest, to be examined by the methods of computed tomography, often occupies only a small fraction of the area of a plane extending through the body. For example, a radiologist may only be interested in determining abnormalities within a single body organ. A disadvantage of the prior art method and apparatus is that an entire body plane must be scanned before local values of the attenuation coefficients in a region of interest can be calculated. This is due to the fact that attenuation of the X-ray beams access along the entire beam path and affects the computation of attenuation coefficients at every point in the plane. Thus, if a limited region of interest were totally scanned and the surrounding body areas were only partially scanned (to the extent they were included in the scanning of the region of interest) features in the partially scanned region would produce image artifacts which would significantly distort computed values within the region of interest. Furthermore, severe restriction is placed on the stability of prior art X-ray tube and detector systems and upon the mechanical precision of the scanning devices since consistent data must generally be obtained over the entire scan time in order to accurately compute local attenuation coefficient values. Problems of reconstruction may similarly arise in regions of the body which are subject to motion during a scan.
A scanning motion consisting of translation followed by separate rotation is usually clumsy and subject to mechanical vibration and wear. Because of the mechanical problems involved, it is often difficult to speed the sequence of translation and rotation movement to reduce scanning time. Further problems are reated to the complexity of prior art computer programs necessary for reconstruction and the sophistication of the programs that are required.